Currently, transistors used in power electronic devices are mainly traditional silicon devices, such as Si CoolMOSs, Si IGBTs, Si MOSFETs and the like. However, with rapid development of power device technology, power devices based on conventional Si and the second-generation semiconductor materials cannot meet the requirements of high speed and low loss in practical applications. Instead, SiC power devices have drawn high attentions in the field due to advantages of their material. Meanwhile, the third-generation semiconductor devices with wide band gaps, in particular, gallium nitride power devices, have become a hot topic of research and have been widely used in fields of radio frequency and power electronics. This is because, on the one hand, that gallium nitride is a kind of semiconductor material having a wide band gap which has a critical breakdown electric field 10 times larger than that of silicon material and correspondingly has a property of withstanding high voltages, and, on the other hand, that a two-dimensional electron gas channel is capable of providing a very small on-resistance so that power loss of a switching device can be reduced. Therefore, planar transistors based on aluminum gallium nitride/gallium nitride heterostructures have become an important research object in the field.
In practical applications of high-voltage power devices, in order to ensure safety of operating systems and application environments, normally-off type devices, i.e. devices having no output current when zero voltage is applied to gate electrodes thereof, are widely used. However, due to characteristics of the aluminum gallium nitride/gallium nitride heterojunction material, depletion type devices are more easily realized.
Currently, in order to produce a normally-off type gallium nitride device, as described in U.S. Patent Publication No. US20140167822 A1, a solution is to connect a high-voltage depletion type device and a low-voltage enhancement type device with cascade circuits. In detail, a source electrode of the high-voltage depletion type device is connected to a drain electrode of the low-voltage enhancement type device via a wire, and a gate electrode of the high-voltage depletion type device and a source electrode of the low-voltage enhancement type device are connected via a wire and function as a source electrode of the semiconductor package structure collectively. Therefore, a drain voltage of the low-voltage enhancement type device becomes a negative gate voltage of the high-voltage enhancement type device, thereby automatically providing a negative bias voltage necessary to achieve depletion type shutdown operations.
However, in the above-mentioned solution, the wires used in interconnection among the electrodes, e.g. the source electrodes, the drain electrodes and the gate electrodes, introduce parasitic inductance and parasitic resistance, which reduces switching speeds of signals and thus adversely affects the characteristics of high frequency and high speed of the gallium nitride power devices. Therefore, it is required to minimize the parasitic inductance and the parasitic resistance introduced in the cascade circuits.